1. Field of the Invention
The present invention relates to a method for driving a liquid crystal display panel, and more specifically to a liquid crystal display driving method capable of displaying a halftone image.
2. Description of Related Art
Liquid crystal displays are increasingly used as a display in personal computers, work stations, word processors, and the like, since they have a feature of being compact and having low power consumption.
As means for displaying an image with a plurality of gradation levels, a frame thinning out system has been known, in which a unitary period is composed of a plurality of continuous frames which can have different brightness, so that a halftone can be displayed. For example, this frame thinning out system is disclosed in European Patent Publication EP-A-0400992, the disclosure of which is incorporated by reference in its entirety into this application.
Here, a liquid crystal display driving method for displaying a halftone image in accordance with a conventional frame thinning out system, will be described with reference to FIGS. 1A to 1C and 2. FIGS. 1A to 1C show a change in brightness for images having different gradation levels, and FIG. 2 illustrates a distribution of brightness in a display region composed of pixels arranged in two rows and two columns.
FIG. 1A shows a change in brightness for an image having the gradation level 1, and FIG. 1B shows a change in brightness for an image having the gradation level 2. FIG. 1C shows a change in brightness for an image having the gradation level 3. Here, each of the gradation levels 1 and 3 is a reference brightness, and in the case of displaying an image, having the gradation level 1 or 3, the brightness is constant over all frames.
On the other hand, in the case of displaying an image having the gradation level 2, the brightness for the gradation level 1 and the brightness for the gradation level 3 are alternately displayed, frame by frame, as shown in FIG. 1B.
The frame frequency is 60 Hz in a display for a television, a personal computer, etc. In the ease of displaying an image having the gradation level 2, the brightness is caused to change, frame by frame, as shown in FIG. 1B. Therefore, a flicker having a frequency of one haft of one frame frequency occurs, so that the quality of the display is deteriorated.
As a means for suppressing this flicker, one method is known in which the brightness levels 1 and 3 are displayed in adjacent pixels, respectively, in each frame, as shown in FIG. 2, so that the flicker component is spatially equalized so as to be made quiet. Specifically, the pixels at positions of a row "n" and a column "m" and a row "n+1" and a column "m+l" are driven in a phase starting from "a" in FIG. 1B, and the pixels at positions of a row n+1" and a column "m" and a row "n" and a column "m+1" are driven in a phase starting from "b" in FIG. 1B.
In the meanwhile, the liquid crystal has the nature that if a voltage of the same polarity is applied for a long period of time, the liquid is deteriorated. Therefore, it is an ordinary practice to adopt an alternating current driving for the liquid crystal display.
Referring to FIG. 3, there is shown an example of the actual alternating current driving voltage waveform in the case of changing the brightness as shown in FIG. 1B. Here, it is assumed that the liquid crystal is in a normally white mode, and therefore, a high voltage is applied for the brightness level 1, and a low voltage is applied for the brightness level 3. Accordingly, in the case of displaying the image at the gradation level 2, the high voltage for the brightness level 1 is applied for a first frame (frame 1), and then, the low voltage for the brightness level 3 is applied for a second frame (frame 2). This voltage application pattern is repeatedly performed, so that the a halftone image having the gradation level 2 can be displayed.
Furthermore, if a dot-inverting driving method is adopted in which the polarity of the voltage applied to adjacent pixels is made opposite to each other, the flicker can be adequately suppressed.
The halftone display method as shown in FIG. 3 of displaying one halftone by use of two frames, is called a two-frame thinning out system.
However, in the halftone display method in accordance with the conventional two-frame thinning out system, the voltage applied in the odd-numbered frames (Frames 1, 3, 5, . . . ) is asymmetric to the voltage applied in the even-numbered frames (Frames 2, 4, . . . ). Therefore, a direct current voltage is applied to the liquid crystal, with the result that an image sticking occurs and furthermore, the lifetime of the liquid display is reduced.